The technology of micro electromechanical systems (MEMS) originates from technology developed over decades in the fabrication of silicon integrated circuits. MEMS permits the fabrication of large arrays of microactuators that can serve as mirrors, valves, pumps, etc. for a variety of applications. Although the invention is not so limited, an important application is an array of tiltable mirrors integrated in a single substrate and used for switching of a large number of optical beams. Each mirror is part of a separately controlled actuator. These actuators are typically electrostatic in nature and require actuation voltages near 100V to operate.
An example of one cell of an electrostatically controlled MEMS array is illustrated in plan view in FIG. 1 and in cross-sectional view in FIG. 2. The cell is one of many such cells arranged typically in a two-dimensional array in a bonded structure including multiple levels of silicon and oxide layers. The cell includes a gimbal structure of an outer frame 110 twistably supported in a support structure 112 of the MEMS array through a first pair of torsion beams 114 extending along and twisting about a minor axis. The cell further includes a mirror plate 116 having a reflective surface 117 twistably supported on the outer frame 110 through a second pair of torsion beams 118 arranged along a major axis perpendicular to the minor axis and twisting thereabout. In the favored MEMS fabrication technique, the illustrated structure is integrally formed in an epitaxial (epi) layer of crystalline silicon. The process has been disclosed in U.S. Provisional Application, Ser. No. 60/260,749, filed 10 Jan. 2001, incorporated herein by reference in its entirety.
The structure is controllably tilted in two independent dimensions by a pair of electrodes 120 under the mirror plate 116 and another pair of electrodes 122 under the frame 110. The electrodes 120, 122 are symmetrically disposed as pairs across the axes of their respective torsion beams 118, 114. A pair of voltage signals VA, VB are applied to the two mirror electrodes 120, and another pair of voltage signals are applied to the frame electrodes 122 while a common node voltage signal VC is applied to both the mirror plate 116 and the frame 110. The driving circuitry for these and similar voltage signals is the central focus of this invention.
Horizontally extending air gaps 124, 126 are formed respectively between the frame 110 and the support structure 112 and between the mirror plate 116 and the frame 110 and overlie a cavity or vertical gap 128 formed beneath the frame 110 and mirror plate 116 so that the two parts can rotate. The support structure 112, the frame 110, and the mirror plate 116 are driven by the common node voltage VC, and the frame 110 and mirror plate 116 form one set of plates for variable gap capacitors. Although FIG. 2 illustrates the common node voltage VC being connected to the mirror plate 116, in practice the electrical contact is made in the support structure 112 and electrical leads are formed on top of the torsion beams 114, 118 to apply the common node voltage signal to both the frame 110 and the mirror plate 116, which act as top electrodes. The electrodes 120, 122 are formed at the bottom of the cavity 128 so the cavity forms the gap of the four capacitors, two between the bottom electrodes 118 and the frame 110, and two between the bottom electrode 120 and mirror plate 116.
The torsion beams 114, 118 act as twist springs attempting to restore the outer frame 110 and the mirror plate 116 to neutral tilt positions. Any voltage applied across opposed electrodes exerts a positive force acting to overcome the torsion beams 114, 118 and to close the variable gap between the electrodes. The force is approximately linearly proportional to the magnitude of the applied voltage, but non-linearities exist for large deflections. If an AC drive signal is applied well above the resonant frequency of the mechanical elements, the force is approximately linearly proportional to the root mean square (RMS) value of the AC signal. In practice, the precise voltages needed to achieve a particular tilt are experimentally determined.
Because the capacitors in the illustrated configuration are paired across the respective torsion beams 114, 118, the amount of tilt is determined by the difference of the RMS voltages applied to the two capacitors of the pair. The tilt can be controlled in either direction depending upon the sign of the difference between the two RMS voltages.
As shown in FIG. 2, the device has a large lower substrate region 130 and a thin upper MEMS region 132, separated by a thin insulating oxide layer 134 but bonded together in a unitary structure. The tilting actuators are etched into the upper region, each actuator suspended over the cavity 128 by several tethers. The electrodes are patterned onto the substrate, which can be an application specific integrated circuit (ASIC), a ceramic plate, a printed wiring board, or some other substrate with conductors patterned on its surface. The actuators in the upper region form a single electrical node called the “common node”. Each actuator is suspended above four electrodes, each electrode being isolated from every other electrode. To cause the actuator to tilt in a specific direction, an electrostatic force is applied between the actuator and one or more of its electrodes by imposing a potential difference between the common node and the desired electrode. Each actuator has two pairs of complementary electrodes, one causing tilt along the major axis and the other causing tilt along the minor axis. Fabrication details are supplied in the aforementioned Provisional Application No. 60/260,749.
One drawback of electrostatic actuation used for this micromirror is a phenomenon known as “snapdown”. Because electrostatic force is inversely proportional to the distance between the electrodes, there comes an angle at which the attractive force increases very rapidly with greater electrode proximity. Beyond this angle, a small decrease in distance leads to an enormous increase in force, and the electronic control loop becomes unstable, causing the electrodes to snap together. With such an actuator in which the electrodes comprise a flat plate suspended over a cavity by small tethers, a rule of thumb states that the plate will begin to snap down at a deflection corresponding to approximately four ninths the depth of the cavity. Hence, in order to achieve a deflection of θ at the end of the cantilever, the cavity must be approximately 2.25 θ deep. Electrostatic MEMS mirror arrays have been used as video display drivers, but they operated at two voltage levels, zero and full snap-down. In contrast, the mirrors described above must be nearly continuously tiltable over a significant angular range.
Optical constraints determine the deflection distance requirement for the electrostatic micromirror. The RMS voltage level required for a given amount of deflection results from a combination of actuator size, tether spring constant, and cavity depth. The cavity depth required to avoid snapdown generally dictates the use of relatively high voltages, typically in excess of 40V, the upper limit for many standard IC processes. The generation of such voltages requires an electronic system composed of high-voltage (HV) semiconductor components, either off-the-shelf or customized, which are fabricated by specialized HV processes, such as the HVCMOS process available from Supertex, Inc.
The application for which the invention was developed requires a 12×40 array of micromirrors, and the mirrors must be independently tiltable in both directions along two axes. Each tilt axis requires its own actuator pair so the driver array is 24×40. The size of the array is dictated by the switching of 40 wavelength-separated channels in a wavelength division multiplexing (WDM) optical network being switched between 6 input fibers and 6 output fibers with a folding mirror optically coupling paired input and output mirrors. Switching is accomplished by selective tilting about a major axis; and, power tuning by selective tilting about a minor axis. The MEMS structure accomplishes bi-directional tilt using two electrodes that are symmetrically placed about the central tether of each axis. Hence, there are four electrodes per microactuator, for a total of 3840 electrodes that must be independently controlled. Optical techniques such as “interleaving” may be used to split the array into two 12×40 chips, but even with this amelioration, each MEMS chip will have 1920 high-voltage inputs and outputs (I/Os). While I/O counts of several thousand are commonplace in certain low-voltage digital technologies such as memories. But, when the inputs here are high-voltage analog signals, as in the described mirror switching array, high I/O counts present a significant packaging problem.
Conventional methods for silicon chip I/O include wire bonding and die-to-substrate attachment known as “flip-chip”. It is generally accepted that wire bonding becomes impractical at about 800 I/O's, due to the large chip perimeter required to contain the bond pads. Integrated circuits with higher I/O counts are typically attached to a substrate with solder bumping, and signals are routed to discrete drivers that are flip-chip bonded to the same substrate, but this solution becomes difficult in the intended application due to the very large number of high-voltage (HV) signals and the size of conventional HV circuitry.
MEMS actuators often exhibit a charging effect that builds up over time and, when the driving voltage is DC, eventually disables operation. Charging therefore dictates that the driving voltage has alternating polarity with zero DC bias. Also, MEMS microactuators may display significant operational variation from actuator to actuator or the operation may depend upon environmental conditions.